Several hundred apertures are drilled and tapped in a typical automobile engine and the various components associated with the engine. Additional apertures are required when other drive train components are included, such as suspension components, various brackets and levers, frame and sheet metal parts and the like. The process of drilling apertures and tapping or roll head formed threads in engine and drive train components does not result in perfect parts one hundred percent of the time. Quality control measures are required to test and verify the accuracy of the apertures and threads, either through a statistical sampling methodology, or through one hundred percent inspection of each drilled and tapped aperture or roll head formed thread. In many manufacturing environments, one hundred percent testing of all drilled and tapped apertures or roll-formed threads is impossible due to time constraints. In such instances, an aperture may be tapped with less than the minimum number of threads, or a damaged tap may create poorly formed threads, or worse yet, an aperture may not get tapped at all. In some instances, a tap can break off in the aperture. When such a component is transferred to the next operation or department, or worst of all, to the customer, substantial problems and costs inevitably result.
Relatively few machine builders have recognized the need for one hundred percent thread inspection following tapping operations, and most leave the inspection responsibility to the engine and drive train component manufacturers. Component manufacturers have dealt with the need to inspect threaded apertures in a variety of ways ranging from visual inspection by an operator to various forms of automated systems. Such automated systems are typically retrofitted as part of the tapping equipment, or immediately following the tapping operation as a separate step. If tapping equipment is originally equipped with thread detection, or if retrofitted, no loss in productivity occurs. If threads are detected off the production line, additional effort may be required.
With an increasingly quality driven environment at all levels of manufacturing in the world, the first efforts to inspect threaded apertures involve visual inspection by an operator of bolts or gauges actually threaded into the aperture as part of the inspection process. It quickly became evident that the human element could not be relied on or simply could not visually perform the necessary inspection due to lapse of attention, aperture depth, and so forth. Furthermore, threading a gauge into every aperture by hand was impractical at required production rates as well as relying on the human element to both read the difficult scales on such devices and to make consistent judgements as to acceptable threading torque thresholds. Enormous manpower was required to thread a bolt into every aperture by hand and this proved economically infeasible. It was recognized that a fast, consistently reliable approach was needed to inspect one hundred percent of the apertures for proper threads. Several contact and non-contact methods were developed involving quite different technologies ranging from mechanical feelers to reflective light to airflow/back pressure to electromagnetic inspection.
Mechanical feelers are occasionally used to contact the inside edge of the aperture in order to sense movement when the feeler is moved actually within the aperture. This method determines that there are threads present on at least one side of the aperture, but does not determine much more than that. Therefore, this method has limited applications, and is not desirable for most inspection purposes.
Reflected light is another method used to detect threads. This is a non-contact, relatively quick inspection technique. It typically uses a sender/receiver probe that can be placed at an angle close to perpendicular to the face of the threads. As a light beam is directed toward the threads, the newly machined or formed surface reflects the light back to the receiver. If a sufficiently high reflectivity is present, a determination is assumed that a machined or formed surface representing threads is present in the aperture with the added assumption that the tap or roller former did the required work. With the ever present abundance of cutting fluids and coolants as well as washing fluids, both the sender and receiver units can degrade in performance. As both sending optics and receiving optics become obscured degrading optical transparency. Varying levels of reflectivity can affect detection capability when threads have fluids or oils deposited on the threads. Also, looking at only a small portion of the threaded aperture and assuming that properly formed threads are present in the rest of the aperture is not accurate in many cases, and does not form an adequate basis for accepting the aperture.
Another approach involves the use of an airflow/back pressure measurement. In this method, a probe is inserted into the aperture and injects air into the aperture. A sensor measures the back pressure of this flow as the air escapes. The turbulence created by the presence of threads causes an increase in back pressure over the back pressure present in the absence of threads, and thus the presence or absence of threads can be inferred. These systems are not adept at determining the actual number of threads to any degree of accuracy.
With the advent of smaller, better performing cameras and software, vision has been adapted for use in detecting the presence of threads in apertures. The environment of cutting fluids and oils on the cutting threads and within the aperture has a pronounced effect on the sensitive lighting requirements of the vision systems. In addition, not all threads are typically viewed with a vision system, and the assumption is made that the unseen threads are present and properly formed.
Another non-contact approach discloses the use of electromagnetic sensing using eddy currents. A probe having a single coil is excited at a specific frequency and is positioned in the aperture. The inductance of the coil plus the threaded aperture is sensed and compared to the inductance of the coil plus a known conforming threaded aperture. Since the inductance of the coil and aperture combination is affected by the combined geometry, among other things, similar geometries will exhibit similar inductance levels. By alternating current through a coil at a specific frequency and amplitude, and bringing the coil within close proximity of a part made of an electrically conductive material, the coil-part combination exhibits a combined inductance (measured in henries) as electrical currents known as eddy currents are induced in the surface of the part. The inductance of this part-coil combination is affected by primarily four characteristics of the material: microstructure/hardness; chemistry; temperature; and geometry. Since non-destructive testing using eddy currents is a comparative method, by placing the coil in the same position relative to another part, the inductance of this second combination is compared to the first one and differences can be detected. If the parts are of the same material at the same temperature, geometry differences can be very effectively detected. Thus, a tester using eddy current technology is calibrated using a known conforming part or master and then is used to detect differences in other materials relative to the geometry. Depending on the sophistication of the electronics sensing these inductance differences, more than just the presence or absence of threads can be detected. A simple, un-tuned type probe transducer circuit can, under ideal conditions, detect threaded conditions where as little as two or three threads are missing. Furthermore, the probes are unaffected by cutting fluids and coolants. The method is non-contact. The method detects 360° of geometry and is very quick to perform. Since this method involves inspecting the aperture at a discreet position within the aperture, the probe may inspect only those threads in the immediate vicinity of the coil on the probe, and ignores the other areas of the aperture that are supposed to be properly threaded.
Another use of eddy current technology involves the use of significantly different electronics and a tuned transducer probe. This approach was developed within the last few years and is purported to have between three to ten times the sensitivity compared to the un-tuned version. The probes are unaffected by cutting fluids. The method is non-contact, inspects in 360° of geometry, and is very quick to perform, approximately less than 0.1 second for the actual electronics and one to two seconds for a total cycling of the fixture actuator. The probe may not inspect threads in all parts of the aperture depending on the depth of the aperture. This method has the added advantage of being able to reliably detect one thread missing conditions in many applications due to the greater sensitivity. In most applications, the probe is advanced into the aperture while being held in a fixture with the part properly located. The fixture can use pneumatic actuators for the purpose of quickly moving the probe into and out of the aperture as controlled manually or via a programmable logic controller. The probe is positioned at the optimal depth within the aperture during each inspection in order to maximize the performance of the tuned transducer probe.
Another recent development in profiling the threaded aperture is an analog eddy current signal obtained as a function of depth into the aperture usually measured from the pierced surface. As the probe first enters the aperture, then passes through such features as counter-bores, then the threaded area itself, then into the unthreaded area of the pilot hole, the signal is continuously compared against an expected profile of a properly formed aperture. The probing coil can be concentric with the aperture centerline or it can be at 90° to the aperture centerline. In this second situation, the process gives up the ability to detect features in a full 360° of geometry, but can easily detect individual threads and provide the ability to count the threads. By alternating current (A.C.) coupling the probe, part to part variability challenges can be effectively negated. However, when threads need to be detected for a full 360° geometry, another technique must be relied on.